The invention relates to an internal-combustion engine having an exhaust gas system that is arranged at an exhaust gas outlet port and has at least one secondary air injection device for injecting secondary air into the exhaust system between the combustion chamber and an emission control system arranged in the exhaust system.
In “Wikipedia”, the free encyclopedia, a secondary air injection system, for example, is described as follows: In the case of Otto engines, the secondary air injection system is used for complying with legal emission standards and laws. It consists essentially of the secondary air pump and the secondary air valve. In the cold starting phase, the system is used for oxidizing the exhaust gas constituents HC (hydrocarbons) and CO (carbon monoxide). In addition, it very rapidly heats the catalytic converter to the operating temperature. By way of the lambda probes, the engine control unit recognizes the beginning effect of the catalytic converter and subsequently switches off the secondary air pump. A secondary air injection system is becoming increasingly important in order to be able to comply with increasingly stricter exhaust gas standards and laws.
1. Mode of Operation:
As a result of the injection of secondary air, HC- and CO-pollutants are reduced in the cold-start period and in the warm-up period of the engine (λ≦1, rich fuel/air mixture) by means of thermal afterburning. In these engine operating states, the controlled three-way catalytic converter will not yet be fully operable.
2. General State of the Art:
A secondary air injection (SA injection) is used for obtaining, for example, the SULEV (Super Ultra Low Emissions Vehicle) exhaust gas certification of the State of California and for observing other very strict exhaust gas limit values for Otto engines. In this case, immediately after the cold start, ambient air is introduced by way of a secondary air pump (SA pump) for a certain time into the hot exhaust gas flow of the internal-combustion engine in order to initiate subsequent reactions, as indicated above. These subsequent reactions cause the oxidation of products of partial combustion (HC, CO), which, on the one hand, results in a reduction of the HC- and CO emissions and, on the other hand, by way of the heat of this subsequent reaction, in a rapid reaching of the conversion temperature of the catalytic converter (so-called catalytic-converter heating). This secondary air injection usually takes place, as close to the exhaust valves as possible, into the exhaust gas outlet ports within the cylinder head in order to further the initiation of the subsequent reactions as a result of the high exhaust gas temperatures at the injection site. During the catalytic-converter heating, the ignition angle is usually set to late (i.e., ignition after the ignition dead center (top dead center) of the piston) in order to intentionally reach a low efficiency of the internal-combustion engine but reach particularly high exhaust gas temperatures. If a camshaft adjustment is possible, the position of the camshaft(s) during the secondary air injection is derived from the requirements of the internal-combustion engine start and the later transition to the starting movement, so that, at most, moderate overlaps of the intake and exhaust valves will occur, resulting in low residual gas contents in the combustion chamber of the internal-combustion engine. The internal-combustion engine operation with the secondary air injection and the late ignition angle will be terminated when certain limits are exceeded, for example, of the time since the start of the internal-combustion engine, the temperature of the catalytic converter and/or the internal-combustion engine, and/or of the vehicle speed.
One disadvantage of this technique is, for example, the one-sided optimization of the HC/CO emissions before the catalytic converter light-off without taking into account the conceivable additional advantageous potentials with respect to the exhaust gas temperature and the NOx emissions. Particularly in the case of internal-combustion engines having an exhaust gas turbocharger (ATL), a very high exhaust gas enthalpy flow is required for reaching the SULEV limit values in order to rapidly heat the catalytic converter because the turbine of the exhaust gas turbocharger acts as a heat sink and has to be compensated.
For providing this exhaust gas enthalpy flow, the internal-combustion engine has to be operated at a very late ignition angle for achieving very high exhaust gas temperatures, which, in turn, requires a high cylinder charge with an inlet gas/fuel mixture for overcoming the internal friction of the cold internal-combustion engine and for supplying consuming devices, such as an air-conditioning compressor or a power steering pump. Under these operating conditions, a considerable formation of nitrogen oxides (NOx) will occur even with an oxygen deficiency in the cylinder (combustion chamber), which nitrogen oxides (NOx) may, for example, slightly exceed the SULEV limit values.
It is an object of the present invention to avoid the above-described disadvantages.
This and other objects are achieved by an internal-combustion engine having at least one combustion chamber and having an exhaust system that is arranged at an exhaust gas outlet port and has at least one secondary air injection device for injecting secondary air into the exhaust system between the combustion chamber and an emission control system arranged in the exhaust system. A residual gas content in the combustion chamber is changeable during operation of the internal-combustion engine and, at the exhaust system, the secondary air injection site is located sufficiently far from the combustion chamber that injected secondary air, as a result of back-flowing exhaust gas in the exhaust system, flows back into the exhaust gas outlet port but specifically not back into the combustion chamber.
By means of the further development according to the invention, it becomes possible to observe, for example, the SULEV emission limit values also in the case of internal-combustion engines having many combustion chambers and/or exhaust gas turbochargers, where a SULEV certification has previously not seemed possible. In a further advantageous fashion, the precious metal charging of the catalytic converter can be reduced without endangering the emissions certification in the case of previous SULEV engines, which leads to a clear cost reduction. In addition, the cell density of the catalytic converter can be reduced without endangering the emissions certification, which results in a lower flow resistance and therefore in a lower exhaust back pressure. This leads to achieving higher power and torque values and/or to the elimination of previously required compensation measures for achieving performance equality between standard and SULEV variants of an internal-combustion engine. Furthermore, the secondary air injection site is advantageously transferred out of the cylinder head, whereby identical parts can be used for standard and SULEV variants of an internal-combustion engine. The transfer of the secondary air injection site out of the cylinder further promotes the cooling and stability of the cylinder head in the case of a SULEV design. This results in an increase of the full-load capability and the service life of the cylinder head and the internal-combustion engine respectively.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.